Angle-resolved measurements of resonant two-photon ionization of helium
Using a new experimental method, physicists at the Max Planck Institute for Nuclear Physics in Heidelberg have investigated the resonant two-photon ionization of helium with improved spectral resolution and angle resolution. To do this, they used a reaction microscope developed at the institute, now combined with a high-resolution photon spectrometer for extreme ultraviolet (EUV) light. The measurements were carried out at the Hamburg Free Electron Laser (FLASH), a brilliant radiation source for intense EUV laser flashes. This allows the events from each individual laser flash to be analyzed in terms of photon energy, resulting in spectrally high-resolution data sets.
Helium, as the simplest and most readily available multi-electron system, is ideally suited for fundamental theoretical and experimental studies. The mutual electrical repulsion of the two electrons plays an important role here - it accounts for a good third of the total binding energy. The interaction with photons (light quanta) is of particular and fundamental interest. Researchers from the Max Planck Institute for Nuclear Physics in Heidelberg from the groups led by Christian Ott and Robert Moshammer in Thomas Pfeifer's department have examined in detail the resonant two-photon ionization of helium at the free-electron laser FLASH at DESY in Hamburg.
In this non-linear process, both electrons simultaneously absorb two extreme ultraviolet photons and form a doubly excited state in which both electrons are in a large orbit around the positively charged helium nucleus. The correlated pair dance of the electrons is unstable and their mutual repulsion causes one to leave the atom while the other falls back to the ground state of the now positively charged helium ion - this process is called autoionization. It occurs when the total energy of the photons corresponds to the discrete excitation energy, i.e. there is resonance.
It is advantageous for the understanding that the photons transfer practically no momentum to the atom, but they do transfer an angular momentum, which in turn influences the angular distribution of the electron. For a detailed measurement, the researchers used a reaction microscope (REMI), which allows a kinematically complete detection of both the photoelectrons and the helium ions. However, one fundamental difficulty still had to be overcome: the free-electron laser supplies sufficiently intense ultraviolet radiation, but the energy of the photons has a fairly large bandwidth and the energy range of maximum intensity also varies from laser flash to laser flash.
However, it is precisely this property that has now been exploited: "We used a spectrometer to measure the energy distribution of the photons in each individual shot and then sorted them according to the photon energy with the highest intensity (peak position)," explains lead author Michael Straub. "Synchronized with the REMI signals, we thus obtain spectrally high-resolution data sets that can be digitally tuned over the entire bandwidth.". The resonance was resolved with this trick and the angular distribution of the photoelectrons in the resonance was measured. In a direct comparison with theoretical calculations from the group of Chris Greene (Purdue University), there was good agreement, but there were also deviations in detail.
"These results and the newly developed experimental methodology open up a promising path to researching the fundamental interactions of a few photons with a few electrons," says group leader Christian Ott, summarizing the scope of the work.